MMUSIC Working Group D. Yon
Internet-Draft Dialout.Net, Inc
Expires: January 14, 2005 G. Camarillo
Ericsson
July 16, 2004
Connection-Oriented Media Transport in the Session DescriptionProtocol (SDP)draft-ietf-mmusic-sdp-comedia-08.txt
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Abstract
This document describes how to express media transport over
connection-oriented protocols using the Session Description Protocol
(SDP). It defines the SDP TCP protocol identifier, the SDP setup
attribute, which describes the connection setup procedure, and the
SDP connid attribute, which provides a connection identifier.
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Internet-Draft Connection-Oriented Media July 20041. Introduction
The Session Description Protocol [3] provides a general-purpose
format for describing multimedia sessions in announcements or
invitations. SDP uses an entirely textual data format (the US-ASCII
subset of UTF-8 [10]) to maximize portability among transports. SDP
does not define a protocol, but only the syntax to describe a
multimedia session with sufficient information to participate in that
session. Session descriptions may be sent using arbitrary existing
application protocols for transport (e.g., SAP [8], SIP [9], RTSP
[5], email, HTTP [7], etc.).
SDP [3] defines two protocol identifiers: RTP/AVP and UDP, both of
which represent unreliable connectionless protocols. While these
transports are appropriate choices for multimedia streams, there are
applications for which connection-oriented transports, such as TCP,
are more appropriate. This document defines a new protocol
identifier, TCP, to describe TCP connetions in SDP.
Connection-oriented protocols introduce two new factor when
describing a session: how and when should end points perform the
connection setup procedure. This document defines two new attributes
to describe connection setups: setup and connid.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in BCP 14, RFC 2119 [2] and indicate requirement levels for
compliant implementations.
3. Protocol Identifier
The following is the ABNF for an m= line, as specified by RFC 2327
[3].
media-field = "m=" media space port ["/" integer]
space proto 1*(space fmt) CRLF
This document defines a new value for the proto field: TCP.
The TCP protocol identifier is similar to the UDP protocol identifier
in that it only describes the transport protocol, and not the
upper-layer protocol. An m= line that specifies "TCP" MUST further
qualify the application-layer protocol using an fmt identifier. Media
lines with the TCP protocol identifier are carried using TCP [1].
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It is RECOMMENDED that documents defining new SDP protocol
identifiers that involve extra protocol layers between TCP and the
media itself (e.g., TLS [6] over TCP) start with the string "TCP/"
(e.g., TCP/TLS).
The following sections define the setup and the connid attributes.
While both attributes are applicable to m= lines that use the TCP
protocol identifier, they are not limited to them. These attributes
MAY be used in any m= line which uses a connection-oriented transport
protocol, even if the protocol identifier of the m= line is not TCP.
4. Setup Attribute
The setup attribute indicates which of the end points should initiate
the connection establishment (e.g., send the initial TCP SYN). The
setup attribute is charset-independent and can be a session-level or
a media-level attribute. The following is the ABNF of the setup
attribute:
setup-attr = "a=setup:" role
role = "active" / "passive" / "actpass"
/ "holdconn"
Active: The endpoint will initiate an outgoing connection.
Passive: The endpoint will accept an incoming connection.
ActPass: The endpoint is willing to accept an incoming connection
or to initiate an outgoing connection.
Holdconn: The endpoint does not want the connection to be
established for the time being.
4.1 The Setup Attribute in the Offer/answer Model
The offer/answer model, defined in RFC 3264 [4], provides endpoints
with a means to obtain shared view of a session. Some session
parameters are negotiated (e.g., codecs to use), while others are
simply communicated from one endpoint to the other (e.g., IP
addresses). The value of the setup attribute falls into the first
category. That is, both endpoints negotiate its value using the
offer/answer model.
The negotiation of the value of the setup attribute takes places as
follows. The offerer states which role or roles it is willing to
perform and the answerer, taking the offerer's willingness into
consideration, chooses which roles both endpoints will actually
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perform during connection establishment. The following are the values
that the setup attribute can take in an offer/answer exchange:
Offer Answer
________________
active passive / holdconn
passive active / holdconn
actpass active / passive / holdconn
holdconn holdconn
The active endpoint SHOULD initiate a connection to the port number
on the m= line of the other endpoint. The port number on its own m=
line is irrelevant, and the opposite endpoint MUST NOT attempt to
initiate a connection to the port number specified there.
Nevertheless, since the m= line must contain a valid port number, the
endpoint specifying using the value active SHOULD specify a port
number of 9 (the discard port) on its m= line. The endpoint MUST NOT
specify a port number of zero, except to denote an m= line that has
been or is being refused.
The passive endpoint SHOULD be ready to accept a connection on the
port number specified in the m= line.
A value of actpass indicates that the offerer can either initiate a
connection to the port number on the m= line in the answer or accept
a connection on the port number specified in the m= line in the
offer. That is, the offerer has no preference as to whether it
accepts or initiates the connection and, so, is letting the answerer
choose.
A value of holdconn indicates that the connection should not be
established for the time being.
The default value of the setup attribute in an offer/answer exchange
is active in the offer and passive in the answer.
5. The Connid Attribute
The preceding description of the setup attribute has been in the
context of using SDP to initiate a session. Still, SDP may be
exchanged between endpoints at various stages of a session to
accomplish tasks such as terminating a session, redirecting media to
a new endpoint, or renegotiating the media parameters for a session.
After the initial session has been established, it may be ambiguous
as to whether subsequent SDP exchange represents a confirmation that
the endpoint is to continue using the current media connection
unchanged, or is a request to make a new media connection. The
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media-level connid attribute, which is charset-independent, is used
to disambiguate these two scenarios. The following is the ABNF of the
connid attribute:
connid = "a=connid:" connection-identifier
connection-identifier = token
The connid attribute provides an identifier for the transport-layer
connection used by the m= line. Connid values are meaningful in the
context of a particular m= line. So, different m= lines in the same
session description MAY have the same connid value.
5.1 Offerer Behaviour
Offerers and answerers use the connid attribute to decide whether a
new transport connection needs to be established or, on the other
hand, the existing transport connection should still be used.
When an offerer generates an m= line which uses a connection-oriented
transport, it SHOULD provide such an m= line with a connection
identifier using a connid attribute, unless the application using the
m= line has other means to deal with connection reestablishment. The
connid attribute in an initial offer (i.e., no transport connection
has been established yet) can take any value. This value identifies
the initial connection that the endpoints will attempt to establish.
After the initial offer/answer exchange, any of the endpoints can
generate a new offer to change some characteristics of the session
(e.g., the direction attribute). If such an offerer wants to continue
using the previously-established transport-layer connection for the
m= line, the offerer MUST use the same connid value for the m= line.
If, on the other hand, the offerer wants to establish a new
transport-layer connection for the m= line, it MUST use a new connid
value. This new connid value MUST be different from the current
connid value in use and SHOULD be different than any connid value
used previously in the same m= line.
The connid value in an offer is only compared with the connid
value currently in use. So, having a connid value different than
the one in use is enough to trigger the establishment of a new
connection. Still, it is recommended to use a value different than
all the previous ones used in the m= line to make debugging
easier.
Note that, according to the rules in this section, an offer that
changes the transport address (IP address plus port number) of an
m= line will have a new connid value for this m= line.
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Internet-Draft Connection-Oriented Media July 20045.2 Answerer Behaviour
The connid value for an m= line is negotiated using the offer/answer
model. The resulting connid value after an offer/answer exchange is
the connid value in the answer.
For an m= line, if the offer contains a new connid value (i.e.,
different from the one in use) the answerer MUST use this value in
the answer. If the offer contains the connid value in use and the
answerer wishes to continue using the existing transport-layer
connection, the answerer MUST use this connid value in the answer. If
the offer contains the connid value in use but the answerer wishes to
establish a new transport-layer connection, the answerer MUST use a
new connid value in the answer.
If the connid value for an m= line resulting from an offer/answer
exchange is different than the connid in use so far, the endpoints
SHOULD establish a new transport-layer connection as indicated by the
setup attribute. If a previous connection is still up, the endpoint
responsible for establishing the new connection performing the active
role SHOULD close it as soon as the offer/answer exchange is
completed. It is up to the application to ensure proper data
synchornization between the two connections.
If the connid value for an m= line resulting from an offer/answer
exchange is the same as the connid in use so far, the endpoints
SHOULD continue using the existing connection.
In the past, it was proposed to use the presence of a media-level
SDP attribute as a flag to indicate that a new connection needed
to be established. It was decided not to follow the flag approach
because an offerer whose intent was to signal "no changes" in a
session would need to issue a different offer than the previous
one (i.e., it would need to remove the flag from the m= line). By
using the connid attribute instead, an offerer signals "no
changes" in a session by issuing an identical offer to the one in
use.
6. Connection Management
An endpoint that according to an offer/answer exchange is supposed to
initiate a new connection SHOULD initiate it as soon as the offer/
answer exchange is completed, even if the endpoint does not intend to
immediately begin sending media to the remote endpoint. This allows
media to flow from the remote endpoint if needed.
Typically, endpoints do not close the connection until the session
has expired, been explicitly terminated, or a new connid value has
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been provided for the m= line. Additionaly, specific applications can
describe further scenarios where an end-point may close a given
connection. In case the session is explicitly terminated by one of
the endpoints (e.g., the endpoint sends a SIP [9] BYE), the end point
terminating the session is responsible for closing the
transport-connection.
If an endpoint determines that the transport-connection for an m=
line has been closed and it should be reestablished, it SHOULD
perform a new offer/answer exchange using a new connid value for this
m= line.
Note that the SDP direction attribute (e.g., a=sendonly) deals
with the media sent over the transport-connection, but has no
impact on the transport-connection itself.
7. Examples
The following examples show the most common usage of the setup
attribute combined with TCP-based media descriptions. For the purpose
of brevity, the main portion of the session description is omitted in
the examples, which only show m= lines and their attributes
(including c= lines).
7.1 Passive/Active
An offerer at 192.0.2.2 signals its availability for a T.38 fax
session at port 54111:
m=image 54111 TCP t38
c=IN IP4 192.0.2.2
a=setup:passive
a=connid:1
An answerer at 192.0.2.1 receiving this offer responds with the
following answer:
c=IN IP4 192.0.2.1
m=image 9 TCP t38
a=setup:active
a=connid:1
The endpoint at 192.0.2.1 then initiates the TCP connection to port
54111 at 192.0.2.2.
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Internet-Draft Connection-Oriented Media July 20047.2 Passive/Active with Connection Reestablishment
Continuing the preceding example, consider the scenario where the TCP
connection fails and the endpoints wish to reestablish the connection
for the session. The endpoint at 192.0.2.2 signals this intent with
the following SDP:
m=image 54111 TCP t38
c=IN IP4 192.0.2.2
a=setup:passive
a=connid:2
The new connid value informs the endpoint at 192.0.2.1 that this SDP
represents the intent to establish a new connection for media
transport, rather than continuing with the original connection. If
192.0.2.1 agrees to continue the session using a new connection, it
responds with:
m=image 9 TCP t38
c=IN IP4 192.0.2.1
a=setup:active
a=connid:2
7.3 Actpass/Passive
In another example, an offerer at 192.0.2.2 signals its availability
for a T.38 fax session at TCP port 54111. Additionally, this offerer
is also willing to set up the media stream by initiating the TCP
connection:
m=image 54111 TCP t38
c=IN IP4 192.0.2.2
a=setup:actpass
a=connid:3
The endpoint at 192.0.2.1 responds with the following description:
m=image 54321 TCP t38
c=IN IP4 192.0.2.1
a=setup:passive
a=connid:3
This will cause the offerer (at 192.0.2.2) to initiate a connection
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to port 54321 at 192.0.2.1.
8. Security Considerations
See RFC 2327 [3] for security and other considerations specific to
the Session Description Protocol in general.
An attacker may attempt to modify the values of the connid and setup
attributes to have endpoints reestablish connections unnecesaryly or
to keep them from establishing a connection. So, it is STRONGLY
RECOMMENDED that integrity protection be applied to the SDP session
descriptions. For session descriptions carried in SIP [9], S/MIME is
the natural choice to provide such end-to-end integrity protection,
as described in RFC 3261 [9]. Other applications MAY use a different
form of integrity protection.
9. IANA Considerations
This document defines two session and media level SDP attributes:
setup and connid. Their formats are defined in Section 4 and Section5 respectively. These two attributes should be registered by the IANA
on
http://www.iana.org/assignments/sdp-parameters
under "att-field (both session and media level)".
This document defines a proto values: TCP. Its format is defined in
Section 3. This proto value should be registered by the IANA on
http://www.iana.org/assignments/sdp-parameters
under "proto".
10. Acknowledgements
Jonathan Rosenberg, Rohan Mahy, Anders Kristensen, Joerg Ott, Paul
Kyzivat, Robert Fairlie-Cuninghame, Colin Perkins, and Christer
Holmberg provided valuable insights and contributions.
11. References11.1 Normative References
[1] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
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